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All I had to do was take off the 2 bolts holding the housing down, lift the housing and hose out of the way...Ensure the block is clean, pop in the new one, tighten it up...Refill the fluids lost...Good to go....

Mine was stuck in there pretty good, so just used a long flat screwdriver to pop it out...

Ensure that the gasket on the thermostat is seated proper beofre you start bolting it down...Should not take more than about 15 minutes...

Bought the blazer 2 months ago. When I test drove it the heat worked great. Now, it is getting cold here and no heat. It is not icy cold air coming out but it is lukewarm/chilly at best. Not sure why it is not warm all of a sudden and not sure what to try first. Have had some people tell me to replace thermostat, others say to flush the heater core. Any suggestions on what could be wrong would be greatly appreciated!Thanks!

Why not do the thermostat andheater core flush together? Here's a good tutorial for the core flush. It might be wise to go ahead and change out the anti-freeze coolant completely if you don't know how old it is.